In recent years, as a next-generation high-capacity solid nonvolatile memory that can perform high-speed reading/writing and low-power consumption operations, a magnetic random access memory (which will be referred to as an MRAM hereinafter) utilizing a magnetoresistance effect of a ferromagnetic material has attracted a great deal of interest. In particular, a magnetoresistive element having a ferromagnetic tunnel junction has drawn attention since it was discovered that it has a high magnetoresistance change ratio.
The ferromagnetic tunnel junction basically has a three-layer laminated structure including a storage layer having a variable magnetization direction, an insulator layer and a fixed layer that faces the storage layer and maintains a predetermined magnetization direction. When a current is flowed through this ferromagnetic tunnel junction, the current tunnels through the insulator layer to flow. At this time, the resistance in the junction unit changes depending on a relative angle of the magnetization directions of the storage layer and the fixed layer, and it takes a minimal value when the magnetization directions are parallel whilst it takes a maximal value when these directions are anti-parallel.
This change in resistance is called a tunneling magneto-resistance effect (which will be referred to as a TMR effect hereinafter). When actually using a magnetoresistive element having a ferromagnetic tunnel junction as one memory cell, a parallel state and an anti-parallel state of magnetization (i.e., a relative minimum and a relative maximum of resistance) of the storage layer and the fixed layer are associated with binary information “0” and “1”, respectively, thereby storing information.
In regard to writing of stored contents in the magnetoresistive element, there is known a magnetic field write system by which a write wiring line is arranged near a memory cell and a magnetization direction of a storage layer alone is reversed by using a current magnetic field produced when flowing a current.
However, when an element size is reduced to realize a high-capacity memory, the coercive force (Hc) of a magnetic material constituting the storage layer increases in principle, and hence there is a tendency that a current required for writing is increased as the element is miniaturized. Further, since the current magnetic field from the writing wiring line decreases in principle with respect to a reduction in cell size, achieving both the reduction in cell size and the decrease in writing current required for high-capacity design is difficult in the magnetic field write system.
On the other hand, in recent years, as a write system that overcomes this problem, a write (spin injection write) system using spin momentum transfer (SMT) is suggested. According to this system, a spin polarized current is flowed through a magnetoresistive element to reverse a magnetization direction of a storage layer, and a quantity of spin polarized electrons to be injected can be reduced as a volume of a magnetic layer forming the storage layer is small. Therefore, this system is expected as a write system that can achieve both miniaturization of the element and realization of a low current.
However, when the element is miniaturized to achieve a high capacity, an energy barrier that maintains the magnetization direction of the storage layer in one direction, i.e., the magnetic anisotropic energy becomes smaller than the thermal energy, whereby a problem that a magnetization direction of a magnetic material sways (thermal disturbance) and stored information can be no longer maintained becomes obvious.
In general, since the energy barrier required for the magnetization direction reversal is represented as a product of a magnetic anisotropic constant (magnetic anisotropic energy per unit volume) and a magnetization reversal unit volume, a material having a high magnetic anisotropic constant must be selected to assure resistance against thermal disturbance in a fine element size region.
In an in-plane magnetization type configuration mainly examined in conventional examples, shape magnetic anisotropy is generally utilized. In this case, to increase the magnetic anisotropic energy, countermeasures such as an increase in aspect ratio of a magnetoresistive element, an increase in film thickness of a storage layer, an increase in saturated magnetization of the storage layer and other measures are required. However, when considering the characteristics of the spin injection system, these countermeasures lead to an increase in inversion current, and hence they are not suitable for miniaturization.
On the other hand, utilizing a material having high crystal magnetic anisotropy rather than the shape magnetic anisotropy can be also considered. In this case, however, an easy-axis in an in-plane direction is greatly decentralized within a film surface, and hence an MR ratio (Magnetoresistance ratio) is lowered or incoherent precessional movement is induced, thereby increasing an inversion current. Therefore, this countermeasure is not preferable either.
Furthermore, in the in-plane magnetization configuration, the magnetic anisotropy that arises based on a shape is utilized, and hence the inversion current is sensitive to unevenness in shape. As a result, when the unevenness in shape increases with miniaturization, unevenness in inversion current also increases.
On the other hand, as a ferromagnetic material constituting a magnetoresistive element, using a so-called perpendicular magnetization film having an easy-axis along a film surface perpendicular direction can be considered. When utilizing the crystal magnetic anisotropy in the perpendicular magnetization type configuration, since the shape anisotropy is not used, an element shape can be reduced to be smaller than that in the in-plane magnetization type configuration. Moreover, since the decentralization of the magnetization easy direction can be also reduced, it can be expected that adopting a material having the high crystal magnetic anisotropy enables achieving both the miniaturization and a low current while maintaining the resistance against the thermal disturbance.
Examples of materials used for the perpendicular magnetization film include L1o ordered alloy series (FePt, CoPt and others), artificial lattice series (Co/Pt, Pd), hcp series (CoPt and others), and RE-TM series (Tb—CoFe and others).
In general, the inversion current required to enable reverse magnetization by the spin injection system is dependent on saturated magnetization Ms and a magnetic relaxation constant α of a storage layer. Therefore, to reverse the magnetization of the storage layer by the spin injection of a low current, reducing the saturated magnetization Ms and the magnetic relaxation α is important.
The saturated magnetization Ms can be decreased by, e.g., adjusting the composition of the magnetic material, or adding a nonmagnetic element. However, the decrease in saturated magnetization Ms should not have any adverse effect on other characteristics.
Also, the magnetic relaxation constant α can be decreased by a multilayered film including a magnetic layer having a small magnetic relaxation constant and a perpendicular magnetization film (e.g., any of the above-described material systems) having a large magnetic relaxation constant. Since the capacity will further be increased in the future, however, this measure alone is insufficient to decrease the inversion current.